The following discussion is not an admission that anything described below is common knowledge or citable as prior art.
In some types of oil or bitumen extraction operations, steam is sent into the ground to aid in extracting the oil or bitumen. A large amount of water is required to generate the steam and finding an economically and environmentally acceptable supply of water can be difficult. At least a portion of the steam is recovered as water, called produced water, with the oil or bitumen. The produced water contains oil and other contaminants taken from the ground and would need to be treated to a permit standard before it could be discharged back into the environment. In many operations, it is therefore preferable to re-use the produced water to generate steam, which reduces the amount of water withdrawn from the environment and discharged to the environment.
For example, a steam assisted gravity drainage (SAGD) process may be used to extract bitumen from an oil sands deposit. Steam is pumped into the oil sands to make the bitumen less viscous. The treated bitumen can then be removed, with water created by the condensed steam, from an extraction well. The produced water is contaminated with, for example, oil and other organic contaminants, dissolved solids and silica. These contaminants, particularly the silica, would quickly scale up or otherwise foul any steam generating equipment if the produced water were used to produce steam directly. There is accordingly a need for economical and reliable treatments of the produced water to remove fouling contaminants. Among other difficulties, the quality of the produced water can vary over time. In particular, during upset conditions the unit operations in a treatment train can encounter variations in the concentration of one or more contaminants, particularly oil and grease, of up to an order of magnitude. The quantity of produced water can also increase over time because the ratio of produced water to oil or bitumen recovered tends to increase in time as a reservoir is worked. Because of the possibility of upsets or increases in required throughput, produced water process trains are generally over-sized relative to average or typical conditions, with the extent of the over-sizing being a balance between the cost of building and running an oversized process train compared the risk and cost of downtime or expansion if the process train can not cope with some conditions.